Sensor for capacitive touch pad pointing device

ABSTRACT

The topology and the shape of sensing electrodes ( 4, 31 ) on the Capacity Touch Pad Sensor that allow sensing of small targets are disclosed. Sensor construction utilizes common PCB manufacture technique.

This application claims priority from U.S. appl. No. 60/320,276 filedJun. 13, 2003, which application is hereby incorporated herein byreference for all purposes.

BACKGROUND ART

Modern computing devices typically utilize some form of pointing devicefor interactions of the user(s) with the Operating System (OS) orGraphical User Interface (GUI). The Capacitive Touch Pad (CTP) is wellestablished as the pointing device of choice for Laptop and Notebookportable computers, and other devices.

A Conceptual Capacitive Touch Pad 60 is demonstrated in FIG. 6. Anon-conductive cover that provides galvanic isolation between the user'shand and the Sensor is omitted for clarity here and in FIGS. 1 through4.

Two groups of electrodes 61 and 62 are utilized. Group 61, withelectrodes parallel to the X-axis, is used for determination of the Ycoordinate (according to the system of coordinates depicted as 63).Group of electrodes 62, with electrodes parallel to the Y-axis, is usedfor determination of the X coordinate. When digit 64 is located on ornear the surface of the Sensor, the capacitances 65 between the digit 64and electrodes belonging to group 61 (as illustrated in FIG. 6) allowfor the determination of the position of the digit 64 in the Y-axis. Thecapacitances between the digit 64 and group of electrodes 62 (not shownfor clarity) allow the determination of the position in the X-axis. Itshould be noted that the user's body does not need a galvanic contact toground, and parasitic body capacitance 66 to ground is sufficient forreliable operations, as it is typically several orders of magnitudelarger than the capacitances 65.

It will be appreciated by a person skilled in the art, that the topologyof the electrodes and their exact shape and position will greatly affectthe operations of the Capacitive Touch Pad. The capacitances 65 aredirectly proportional to the area of contact (footprint) between thedigit 64 and groups of electrodes 61 and 62. Therefore, it is desiredthat the areas of the electrodes on the surface of the Sensor be asgreat as possible.

One possible approach utilized in the Prior Art devices is shown in FIG.7. There, the diamond-shaped elements are interconnected into Rows andColumns. The Columns 71 are created by connecting the elements by atrace on the same top layer. The Row elements 72 are joined together bytraces 75 on a second layer. The Row element may have galvanicconnections, through vias between the layers, or may be capacitivelycoupled to the traces 75. The distance between the Rows is illustratedas Spacing 73.

It should be self-evident that if a user touches the pad generating afootprint 74 with diameter less than the spacing 73 and positioned asshown, the footprint will not register on the Rows at all.

In the Prior Art implementations, the ability to sense the small targetsis achieved by increasing the number of Rows and Columns. However, thisapproach necessitates a corresponding increase in the number of signallines on the sensing and controlling circuit, often implemented as asingle Solid-State Integrated Circuit (IC) or a group of ICs, withcorresponding increase in costs.

It is much more preferable to achieve the objective of small targetsensing by some other means, rather than the brute-force approach ofemploying a simple increase of the number of the sensing electrodes.

SUMMARY OF INVENTION

The current invention teaches the topology and shape of sensingelectrodes on the Capacitive Touch Pad Sensor that allow sensing ofsmall targets. One specific implementation shown in FIGS. 1 through 4provides for a 5× improvement for the size of the minimum footprintnecessary for operations of the Capacitive Touch Pad. Thisimplementation uses a common PCB (printed circuit board) manufacturingprocess without the requirement of super-fine lines and spacings, andwithout a need for very-small holes and vias.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows the Artwork for the First (Top) Copper Layer.

FIG. 2 depicts a Fragment of the Magnified Artwork for the First (Top)Copper Layer.

FIG. 3 shows the Artwork for the Second (Internal) Copper Layer.

FIG. 4 demonstrates a Fragment of the Magnified Artwork for the Second(Internal) Copper Layer.

FIG. 5 shows the Composition of the PCB for the Capacitive Touch PadSensor.

FIG. 6 illustrates a Conceptual Capacitive Touch Pad Sensor.

FIG. 7 explains Previous Art implementation of a Capacitive Touch PadSensor.

DETAILED DESCRIPTION

It is desirable for Rows to occupy the whole top area of the CapacitiveTouch Pad, with only small isolation gaps between individual Rows, whenthe measurements of the Y-axis are made; and for the Columns to occupythe whole top area of the Capacitive Touch Pad when the measurements ofthe X-axis are made. However, it is not practical, since the top areamust be shared. The current invention allows for the Rows and Columns tohave minimal gaps of coverage as compared to the Prior Artimplementations.

Referring to FIG. 1 and FIG. 2, the Rows 3 are created by utilizing a“double-comb” pattern. It is desirable to have as many “fingers” aspossible per unit of length, however the limitations of themanufacturing process impose a bound upon the minimum feature size, bothfor the copper conductors and for the spaces between them. The Sensorillustrated in FIGS. 1 through 4 can be produced with a standardlow-cost PCB process often called “8/8”, meaning that both the minimumwell defined copper conductors and the spaces between them are 8 mils(0.008″ or 0.2 mm). A person skilled in the art will immediatelyrecognize that if the Capacitive Touch Pad is required to operate witheven smaller target footprints, a finer PCB processing method could beused, resulting in larger number of “fingers” and finer connectingcopper traces between them.

Rows 2, located at the edges of the Sensor, are constructed with thesame “comb” pattern on one side, and continuous area on the other side,with the combined total areas of each of the “side” Rows 2 approximatelyequal to the total areas of every “regular” Row 3.

Columns are created from individual element 4, which are interconnectedvia the copper traces 31 on the separate layer (illustrated in FIG. 3and FIG. 4).

The “fingers” on the Rows and Columns are interleaved on the Top Layerof the Sensors, allowing for operations with targets having only a smallfootprint.

The Sensor includes annular copper 1 around the electrodes, connected toground potential in normal use. This acts as a shield and a sink forcurrents created by an Electro-Static Discharge (ESD) event, when theuser's body acquires significant charge, and is discharged when the handtouches the Sensor. A non-conductive cover that provides galvanicisolation between the user's hand and the Sensor is omitted for clarityin FIGS. 1 through 4. Therefore, the most likely point of entry for theESD is on the sides of the Sensor, and ESD will be absorbed by thecopper 1 and directed to ground without causing any harm to the circuit.

The Sensor also includes a ground plane 30 on the Second Copper Layerthat shields the sensing electrodes on the Top Layer from the circuitstypically located on the bottom of the PCB. It is also desirable thatthe next Layer after the Second Copper Layer incorporates solid areas ofcopper connected to ground and located under Column traces 31. Usingthis method all of the sensing electrodes on the Top Copper Layer areelectrostatically shielded from the rest of the circuits.

A ground plane 30 incorporates round areas 32 without copper that areused for vias 59 connecting the Rows 2 and 3 to the rest of the circuit.

An example composition of the Sensor's PCB is illustrated in FIG. 5. Itshows four (4) separate Copper Layers 51, 52, 53, and 54; inter-layerisolator/dielectric and adhesive 55; and three types of connecting vias56, 57, and 58. Via 59, although appearing dissimilar, is in fact thesame as via 58, except it does not have any connections from theinternal Copper Layers 52 and 53. Via 59 is created in the same PCBtechnological processing step as via 58.

Layers 51 and 52 are used for the Capacitive Touch Pad Sensor itself,and Layers 53 and 54 are normally used for the circuitry typicallylocated on the bottom of the PCB.

It will be appreciated by a person skilled in the art that Copper Layer53 (the next Layer after the Second Copper Layer) is mostly free forwiring except for the grounded areas under traces 31. This allows foreasy routing of any circuitry located on the bottom of the PCB.

It is self evident that the minimum target footprint 5 (FIG. 2) for thecurrent invention is much smaller then the minimum target footprint 74(FIG. 7) for the Previous Art, under the condition that the spacingbetween the Rows/Columns for the current invention is the same as thespacing for the Previous Art implementation.

1. A capacitive touch pad comprising first and second layers, the firstlayer comprising a non-conductive cover providing galvanic isolation ofthe second layer, the second layer comprising a plurality of row-shapedrow-sensing electrodes and a row-by-column array of column-sensingelectrodes, each column of column-sensing electrodes interconnected byconductive traces, the row-sensing electrodes and column-sensingelectrodes defining interleaved combs therebetween, each comb comprisingat least two fingers.
 2. The capacitive touch pad of claim 1 wherein thefingers are no wider than eight mils.
 3. The capacitive touch pad ofclaim 1 wherein the fingers define spaces therebetween, and the spacesare no wider than eight mils.
 4. The capacitive touch pad of claim 1further comprising a third layer, the second layer lying between thefirst and third layers, the third layer comprising a ground plane. 5.The capacitive touch pad of claim 4 further comprising a fourth layer,the third layer lying between the second and fourth layers, the fourthlayer bearing circuitry.
 6. The capacitive touch pad of claim 1 whereinin the second layer further comprises annular copper around theelectrodes.
 7. The capacitive touch pad of claim 6 wherein the annularcopper is connected to ground potential.
 8. The capacitive touch pad ofclaim 1 further comprising an isolator/dielectric layer between thefirst and second layers.
 9. The capacitive touch pad of claim 4 furthercomprising an isolator/dielectric layer between the second and thirdlayers.
 10. The capacitive touch pad of claim 5 further comprising anisolator/dielectric layer between the third and fourth layers.